What size heater for a 20 gallon tank?
Quick answer: Aqueon's heater chart puts 20 gallons in two bands: 50 watts for 5 to 20 gallons and 100 watts for 15 to 40 gallons. Aqueon does not state a room temperature, so the calculator below uses your room and target temperatures with this site's watts-per-gallon planning estimate.
20 gallon heater calculator
Planning estimate used on this site (not a published standard): 2.5 watts per gallon for a rise of 5°F or less, 3 up to 9°F, 4 up to 15°F and 5 above that, rounded up to the next standard heater size. Same function as the heater wattage guide.
Aqueon heater wattage guidelines
| Heater | Aquarium size |
|---|---|
| 50 watts | 5–20 gallons |
| 100 watts | 15–40 gallons |
| 150 watts | 30–55 gallons |
| 200 watts | 50–75 gallons |
| 300 watts | 65–90 gallons |
Source: Aqueon heater wattage guidelines (PDF).
50 or 100 watts?
Twenty gallons is the top of Aqueon's 50 watt band and inside its 100 watt band. Aqueon's one-page chart doesn't say what room temperature either band assumes. The bigger the gap between your coldest room temperature and your target, the more heat the tank needs, which is what the calculator works out.